Segment Routing leverages a networking source routing paradigm. An ingress node steers a packet through an ordered list of instructions, called segments. Instructions of the ordered list of instructions represent functions to be called at a specific location in the network. A function is locally defined on the node where it is executed and may range from simply moving forward in the segment list to any complex user-defined behavior. Network programming consists of combining segment routing functions, both simple and complex, to achieve a networking objective that goes beyond mere packet routing.
“In-situ” Operations, Administration, and Maintenance (OAM) refers to the concept of directly encoding telemetry information within the data packet as the packets traverse the network or telemetry domain. Mechanisms which add tracing or other types of telemetry information to the regular data traffic, sometimes also referred to as “in-band” OAM, can complement active, probe-based mechanisms such as ping or traceroute, which are sometimes considered as “out-of-band” because the messages are transported independently from regular data traffic. In relation to “active” or “passive” OAM, “in-situ” OAM can be considered a hybrid OAM type. While no extra packets are sent, in-situ OAM (iOAM) adds information to the packets and therefore cannot be considered passive.